1. Field of the Invention
The invention relates to a method of producing superconductors, especially in form of tapes with a relatively high critical current density, by a mechanical deformation provided by means of a tool mechanism comprising movable clamping jaws and hinged brackets and eccentric arms connected to the clamping jaws, which are formed in such a way that the final product has a uniform thickness.
2. Background Art
It is known that high-temperature superconducting tapes can be produced by filling a mixture of metaloxide powders into a metal pipe, whereafter the pipe is deformed by being pulled into a round wire, followed by being extruded, rolled or pressed into a flat tape. Then the tape is heat-treated in such a manner that the powder is sintered to form a coherent, superconducting mass. These mechanical and thermal processes can be repeated in order to improve the superconducting properties, including the critical current density.
Metaloxide powders which can be used are, for instance, YBa.sub.2 Cu.sub.3 O.sub.7, Bi.sub.2-x Pb.sub.x Sr.sub.2 Ca.sub.2 Cu.sub.3 O.sub.y, Tl.sub.2 Ba.sub.2 Ca.sub.2 Cu.sub.3 O.sub.y in polycrystalline form, in which the crystals are slightly plastic and very brittle. The metal coatings can be made of Ag or Ag-alloys which unlike the above mentioned powders, are very plastic and brittle. It is difficult to control a mechanical deformation of composite articles made of such materials, because the materials possess differing material liquidities. A ceramic, superconducting article must, however, possess predetermined structural properties in order to obtain a high critical current density. As few microcracks as possible must be present together with a high degree of texture with the superconducting Cu--O planes parallel to the current direction, and a high degree of uniformity with respect to the density, as well as a good electric interaction between the superconducting grains. It is important for obtaining these features that it be possible to control and optimize the forces having an effect on the article during the mechanical deformation.
It has been demonstrated on small lengths of tape, e.g. ones which are about 2 to 8 cm, in length that the critical current density can be improved by 3 to 4 times, provided the article after the first rolling and heat-treatment is subjected to one or more cycles involving a uniaxial force applied perpendicular to the current direction and followed by a sintering. It is, however, difficult to operate such a procedure as a continuous process, which is presumably necessary for allowing use of the articles for the production of tape-shaped wires.
During rolling of a wire into a tape, the deforming forces can be divided into a pressing force and a displacing force. During rolling, these two-forces are non-homogeneously distributed in the tape with the result that the superconducting ceramics are non-homogeneously compressed. In addition, the displacing force causes microcracks transverse to the current direction in the plane of the tape, either during the rolling or during the following sintering where possible residual tensions are released. The deformation caused by the rolling results in a higher material liquidity in the longitudinal direction than in the transverse direction of the tape. Ceramic crystals already developed can thereby be broken during succeeding rollings with the unfortunate result that microcracks may arise in the transverse direction.
Uniaxial pressing provides a very uniform power effect without noticeable displacing forces in the longitudinal direction of the wire. Such a pressing causes the material to be liquidized substantially only in the transverse direction. In addition, such a deformation reduces the formation of microcracks transverse to the current direction. Uniaxial pressing can, however, not be performed continuously.